The invention relates to the field of satellites, and more specifically to coolers used in geostationary spin-stabilized satellites. The term xe2x80x9cspin-stabilizedxe2x80x9d is used herein to designate satellites that rotate about their own axis in addition to any movement on their orbit.
Equipment on board a satellite, such as a radiation detector, needs to be protected both against solar radiation and against infrared radiation coming from high temperature surfaces, typically at 300 kelvins (K). Proposals have been made to use active coolers; nevertheless such designs require electricity. They are also poorly adapted to spin-stabilized satellites because of feasibility constraints for the mechanical environment generated by the satellite spinning. It is commonplace to find dynamic acceleration levels on spin-stabilized satellites lying in the range about 10 g to 15 g, i.e. 98.1 meters per second per second (msxe2x88x922) to 147.15 msxe2x88x922. Finally, the reliability of such active coolers is limited and can be improved only by providing redundant thermal equipment; such a solution leads to greater mass.
Proposals have therefore been made to use passive coolers. U.S. Pat. No. 4,820,923 describes a passive cooler for a cryogenically-cooled radiation detector which has a toroidal reflective surface. More precisely, the detector is disposed at the end of a tube whose walls are provided with toroidal reflector segments; GB-A-2 115 143 proposes a detector of the same type. U.S. Pat. No. 4,775,792 describes another form of reflector, adapted to protecting a strip of infrared detectors. U.S. Pat. No. 4,150,552 describes an assembly comprising a condenser and a radiation well placed in the vicinity of the detector that is to be cooled; that document mentions various surface treatments for the materials. U.S. Pat. No. 4,121,434 proposes a device for protecting a high resolution radiometer. The device comprises a first conical stage with a shield mounted on a hinge, and a second cylindrical stage located at the end of the cone. U.S. Pat. No. 4,030,316 describes a passive cooler having three stages for cooling an infrared detector. The first stage is frustoconical and reflects solar energy in a single reflection. The second stage is also frustoconical and lies inside the first stage. The third stage is cylindrical and is placed at the end of the second stage. U.S. Pat. No. 3,422,886 also shows a passive cooler with an example that includes a structure made up of a plurality of cones nested in one another. JP-A-4 323 525 describes a cooler formed by a Dewar flask in contact with a supply of coolant. The flask contains a mirror in the form of an ellipsoid of revolution. The device as a whole presents circular symmetry.
A cooler comprising two circular cones was used for the first generation of MeteoSat satellites (MOP). The cooler comprises a sunshade whose function is to protect the cold stages of the cooler from sunlight that is direct or indirect (by reflection on the sunshade); the sunshade is frustoconical in shape with a plane radiator in the end of the truncated cone. The cooler also comprises a conical infrared deflector which is placed at the end of the sunshade in the middle of the radiator thereof. The function of that reflector is to protect the radiator of the detector stage from any direct infrared radiation coming from the sunshade: nearly all of the infrared radiation coming from the sunshade is reflected on the walls of the deflector and is returned outwards after one or more reflections. Finally, the cooler includes a radiator installed at the base of the deflector; the detectors are placed on the rear face of the radiator.
For the cooler of the Seviri instrument of the second generation of MeteoSat satellites, Matra Marconi Space has used a cooler of the same kind, but in which the infrared deflector is in the form of an ellipsoid of revolution having a circular base. The radiator as a whole is still circularly symmetrical. Compared with a dual cone cooler, that solution makes it possible to increase the dimensions of the radiator installed at the base of the defector to a small extent, other things being equal.
A cooler comprising a single circular cone was proposed to the European Space Agency (ETA) by Matra Marconi Space in the context of phase A of the second generation MeteoSat project. That device uses a sunshade as the dual cone cooler; it also comprises two plane radiators disposed at the base of the sunshade. The problem with such a device is that the radiator is not protected from infrared radiation coming from the sunshade. This gives rise to a significant increase in the thermal load on the radiator, thereby reducing the temperature performance of the detector stage.
The invention relates to the problem of performance in passive coolers; in addition, as with all equipment mounted on a satellite, there is also the problem of equipment mass and bulk.
The invention proposes a novel passive cooler which presents improved thermal performance and which makes better use of the space available.
More precisely, the invention provides a passive cooler for a satellite, the cooler comprising a sunshade and a radiator forming the end of the sunshade; the sunshade has an elliptical base and a surface that forms a constant angle relative to the axis of the cooler.
Preferably, the cooler further comprises an infrared deflector at the end of the sunshade together with a radiator forming the end of the deflector. The deflector can also have an elliptical base.
In either case, the ratio of the major axis over the minor axis of the elliptical base is advantageously greater than or equal to 1.1, or even greater than 1.3.
It is possible to provide radiators on the outside face of the sunshade.
In an embodiment, an incident ray in the vicinity of the edge of the sunshade traveling towards the end of the sunshade at the other side thereof, does not penetrate into the deflector.
It is also advantageous for an infrared ray emitted by the sunshade and penetrating into the deflector to leave the deflector after being reflected at least once on the wall of the deflector.
In an embodiment, the angle between the axis of the cooler and the wall of the sunshade is greater than 23.5xc2x0; the angle between the axis of the cooler and the wall of the deflector is close to 10xc2x0.
The invention also provides a satellite including such a cooler; the satellite can be geostationary and spin on its own axis.